Thermoplastic elastomers are elastomeric (i.e., reversibly deformable) polymers that form physical crosslinks which are reversible, for example, by dissolving or melting the polymer. Triblock copolymers having an elastomeric low glass transition temperature (Tg) midblock and hard elevated Tg endblocks are common examples of thermoplastic elastomers. As is well known, such copolymers tend to phase separate, with the elastomeric blocks aggregating to form elastomeric phase domains and the hard blocks aggregating to form hard phase domains. Without wishing to be bound by theory, it is believed that because each elastomeric block has a hard block at each end, and because different hard blocks within the same triblock copolymer are capable of occupying two different hard phase domains, the hard phase domains become physically crosslinked to one another via soft blocks.
Examples of such triblock copolymers are poly(styrene-b-isoprene-b-styrene) (SIS), poly(styrene-b-butadiene-b-polystyrene) (SBS), poly(styrene-b-ethylene/butylene-b-styrene) (SEBS), and poly(styrene-b-isobutylene-b-styrene) (SIBS). Taking SIBS as a specific example, these polymers have proven valuable as drug release polymers in implantable or insertable drug-releasing medical devices such as drug-eluting coronary stents. In addition to their drug release characteristics, SIBS copolymers have been shown to have excellent biostability and biocompatibility, particularly within the vasculature. Moreover, they have excellent mechanical properties for coronary stent applications, including good elasticity and high tensile strength. As a result of their mechanical properties, these polymers are able to undergo crimping and to expand as the stent is expanded, for example.
Despite the desirable qualities of these and other thermoplastic elastomers, there are situations where it would be desirable to improve adhesion between these materials and adjacent materials, particularly metallic materials. For example, good adhesion may be desirable where a polymeric coating is located on the outer (abluminal) surface of a metallic stent. Taking SIBS as an example, as seen from FIG. 1, adhesion between SIBS and stainless steel decreases dramatically as one increases the amount of styrene in the SIBS material from 17 mol % styrene to 24.1 mol % styrene. These tests were performed for film compositions containing from 17 to 51 mole % styrene SIBS (sample thickness 0.15-0.21 mm, N=10) utilizing the ASTM D 903-98 peel adhesion method.